1. Field of the Invention
This invention relates in general to electrooptical distance measurement apparatus and method, such as may be used in field and land surveying. A high precision of measurement may be thus achieved for example for observation of movements of buildings or of the earth's crust, although the invention is not to be limited to these particular applications.
2. Description of the Prior Art
The well known electrooptical measurement of distances is based upon the electrical measurement of the time a beam of light needs to traverse the distance to be measured. If the velocity of light propagation is known the distance is obtained by multiplying velocity by time of propagation. With almost all practical applications light signals are transmitted from one end of the distance and then by means of a reflector at the other end the signals are redirected to their origin. For measuring time of propagation the transmitted light beam receives particular marks. A short signal flash or pulse may be used to measure the time of flight directly as known from the pulse methods, whereas with phase method the propagation time is obtained indirectly by measuring the phase shift a continuously modulated beam returning from the reflector has undergone with respect to the outgoing beam.
Phase measurement is done by one of two different methods, namely the analog and the digital one. Analog phase measurement is done with a phase shifter by shifting the unknown phase with respect to the phase of a reference signal until a known phase difference is observed. The necessary phase shift is read from the phase shifter. As known from CH-Patent No. 596 559 a coaxial delay line with adjustable mechanical length may be well used as a phase shifter for modulation frequencies near 500 MHz, the change in length giving a rather precise value of the corresponding phase shift. Unfortunately such a line has turned out to be too large and it cannot be made small enough for use in field surveying.
Other known methods for direct phase measurement at modulation frequencies around 500 MHz have an insufficient precision for a number of applications. For improving performance it is known to frequency-convert high frequencies of up to 15 MHz to a lower frequency by heterodyning with a beat frequency oscillator, and to effect the phase shift and phase measurement as mentioned above with the converted signals (see A. W. Kondraschkow: "Elektrooptische Entfernungsmessung", Berlin, 1961, pages 173 et seq.). For precision distance measurements at frequencies around 500 MHz however this method has likewise proved to be inadequate.
An object of the present invention is, therefore, to provide a method and an apparatus for electrooptical distance measurement which allow sufficient precision and at the same time a miniaturisation convenient for field use.
The foregoing objects, other objects as well as the numerous advantages of the present invention are set forth in the following disclosure.